1. Field of the Invention
The present invention relates to a semiconductor laser driving circuit used in an optical disc drive, or the like, and specifically to the technique of controlling emission of a semiconductor laser at the time of power-on where the supply voltage is unstable.
2. Description of the Prior Art
An optical disc drive includes a driving circuit for driving a semiconductor laser and an emission control circuit for controlling the emission intensity of the semiconductor laser, or the like. These circuits require power supplies of different voltage levels in many cases. Since the emission control circuit is generally realized by a digital circuit, the supply voltage of the emission control circuit is lower than the supply voltage of the semiconductor laser driving circuit.
When the optical disc drive is powered on, if the semiconductor laser driving circuit is powered on earlier than the emission control circuit, the semiconductor laser driving circuit operates to allow a semiconductor laser to emit light before the emission control circuit enters the normal operation state, resulting in some undesirable problems, such as unnecessary writing in an optical disc, breakage of the semiconductor laser, or the like.
In a conventional laser driving circuit, a Zener diode and a comparator capable of operating at a voltage lower than that of the above emission control circuit are used to construct a circuit for comparing the supply voltage of the emission control circuit with a reference voltage. The logic level of a control signal of the emission control circuit is set such that the semiconductor laser does not emit light when the supply voltage of the emission control circuit is equal to or lower than the reference voltage, whereby the above problems are prevented (see, for example, Japanese Unexamined Patent Publication No. 61-289543 (FIG. 1)).
In a laser driving circuit realized by one chip, if a Zener diode is used for obtaining a reference voltage as in the above conventional technique, it is necessary to fabricate a circuit through a bipolar process, and therefore, it is difficult to realize the laser driving circuit in the form of a MOS device. This means that it is difficult to achieve a low-voltage circuit.
Further, the circuit scale is large because a comparator is used for comparing the supply voltage of the emission control circuit with the reference voltage.
Furthermore, the supply voltage of the emission control circuit cannot be compared with the reference voltage before the supply voltage of the emission control circuit reaches a voltage level at which the comparator can operate. Thus, the above technique does not necessarily entirely prevent the above problems, such as unnecessary writing in an optical disc, etc.